A power performance of the vehicle is governed by a relations among a drive demand such as an opening degree of accelerator, an output amount such as an opening degree of a throttle valve, and a speed ratio. Meanwhile, steering characteristic is governed mainly by an assisting amount of steering angle or steering force. In turn, suspension characteristic is governed by e.g., a damping force of a shock absorber. A behavior of the vehicle is substantially governed by those factors, however, required performance for the vehicle differs depending on driver's preference or intention, road congestion, kind of road such as local road, highway etc. Therefore, control systems configured to select a driving mode out of plurality of modes have been developed. According to the conventional control systems of this kind, a speed change diagram, a second-order characteristic of a throttle opening, a damping force of a shock absorber, a vehicle height achieved by a suspension etc. can be changed depending on driver's choice or preference, by selecting the driving mode.
For example, Japanese Patent Laid-Open No. 2008401742 discloses a control apparatus configured to change conditions for changing a speed ratio in a manner to achieve sporty running, in case degree of quickness required by the driver calculated based on acceleration, opening degree of accelerator, brake pedal force etc. is greater than a threshold value (that is, in case a counter value is greater than a threshold value). According to the teachings of Japanese Patent Laid-Open No. 2008401742, therefore, a vehicle behavior required by the driver can be achieved without selecting the driving mode by the driver.
Meanwhile, PCT international publication WO2011/021634 discloses a vehicle control system configured to control characteristics of driving force, speed ratio, suspension etc. based on a command index representing driving preference of the driver that is obtained based on a synthesized acceleration of the longitudinal acceleration and lateral acceleration.
As described, according to the teachings of Japanese Patent Laid-Open No. 2008401742, the conditions for changing the speed ratio is changed in case the degree of quickness calculated as a counter value becomes greater than the threshold value. This means that the sporty running mode will not be established until the degree of quickness exceeds the threshold value. Therefore, the vehicle may not be driven in accordance with the driver's intention. If such threshold value is set to a larger value, the driving mode will not be shifted to the sporty mode promptly. In this case, therefore, a driving performance required by the driver may not be achieved. To the contrary, if the threshold value is set to a relatively smaller value, the driving mode may be shifted to the sporty mode unintentionally by a temporal acceleration or deceleration carried out to stay coordinated with the other vehicles or traffic condition. This may give driver a feeling of strangeness about a driving performance.